The Universal Mobile Telecommunication System (UMTS) Terrestrial Radio Access Network (UTRAN) is the radio network of a UMTS system, which is one of the third-generation (3G) mobile communication networks. Evolved UMTS Terrestrial Radio Access Network (E-UTRAN), also referred to as Long Term Evolution (LTE) is standardized by 3GPP. Long Term Evolution (LTE) which is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard with High Speed Packet Access functionality to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, lowered costs etc.
An E-UTRAN typically comprises user equipments (UE) 120 wirelessly connected to radio base stations 130A-D as illustrated in FIG. 1. In the E-UTRAN, the radio base stations 130A-D are directly connected to a core network (CN) 100 e.g. via a mobility management entity (MME). In addition, the radio base stations 130A-D are also connected to each other via an interface. The base stations are usually referred to as NodeB in UTRAN and to eNodeB in E-UTRAN. In the E-UTRAN, each radio base station 130A-D uses an Orthogonal Frequency-Division Multiplexing (OFDM) time-frequency grid 140 for data transmission to user equipments within each cell. The OFDM time-frequency grids transmitted from different base station interfere with each other, which reduce channel quality in the E-UTRAN. A part of the OFDM time-frequency grid 140 transmitted from a base station is illustrated in FIG. 1. The grid 140 consists of transmission resource elements of different types. There are for instance transmission resource elements used for data transmission 160, for reference symbols (RS) 150, 180. Hence, the reference symbols may be placed among both the transmission resource elements used for data transmission 160 and among the transmission resource elements 170 used for control signalling. The interference situation for these types of transmission resource may be different. This depends on that the transmission resource elements may be differently power controlled and that the amount of dispersion in a propagation channel and different parts of a frequency band may undergo different fading realizations.
The radio base station needs to have some measure of how “good” the channel is to e.g. determine proper data rate, modulation scheme, and transmit power. Accordingly, the mobile terminal provides a measure of channel quality to the radio base station by means of Channel Quality Indicator (CQI) values that are continuously fed back to the radio base station on an uplink channel. The mobile terminal determines the CQI values based on measurements made on e.g. the common reference symbols (RS) 150 transmitted in the OFDM time-frequency grids from the base station. The noise and interference between the cells are important quantities when estimating e.g. the CQI. The knowledge of the amount of noise and interference is also important to be able to demodulate the information correctly. The common reference symbols (RS) in the OFDM time frequency grid transmitted from each base station may be used to estimate the interference. A received signal “r” may be expressed as r=Hs+n, where “H” is a channel response, “s” represents transmitted symbols and “n” represents unknown noise and interference. It is noted that the noise and the interference on an RS, referred to as I_RS, may be estimated since “s” comprises known symbols and “H” is given by a channel estimator. It is further noted that the interference on data symbols, I_d, also may be measured as soon as the data symbols (s) are detected and that they at this moment may be regarded as known symbols.
A problem is that there is a limited set of reference symbols, and in particular for Multiple Input Multiple Output (MIMO) where a position holding a reference symbol on one antenna is unused for a neighbouring antenna, the statistics of the estimated interference may therefore be very poor. The RS grid, i.e. how the reference symbols are allocated, in case of one transmit (Tx) antenna is illustrated in FIG. 1. Between the cells, the reference symbols are shifted in the frequency domain. For e.g. two Tx antennas only three frequency shifts for common reference symbols exists which results in that not all data interference can be measured. It is also difficult to plan these shifts, since it may be difficult to allocate different shifts to different cells since there are too few of them. There are only three orthogonal patterns for 2×2 MIMO, but there might be more than two interfering cells. Furthermore, the first three OFDM symbols 170 might be affected by control channel interference instead of data interference. The control signalling can occupy up to 3 OFDM symbols (first 3). If the network is time synchronized, the control channels from all cells will overlap. This means that the RS located in the first symbol will be hit by control signalling, while RS located in the 5th OFDM symbol will be hit by data. Since control and data may be transmitted with different power, the interference might be different.
As mentioned above, the control signalling may be differently power controlled than the data, the interference estimate obtained on these common reference symbols may therefore not reflect the interference situation for the data transmission. Also if common reference symbols in a subsequent part of a sub-frame is removed, in which case a dedicated reference symbol will be inserted instead, it might be necessary to measure interference on data symbols instead. Since this in general is a very complex operation, simplifications of the procedure are necessary. It is further noted that when using the interference estimate for calculating e.g. the CQI, it is important to have a well defined measure for the interference estimate. This depends on that a network scheduler is using a reported CQI for resource allocation and it is therefore important that the scheduler knows that all terminals have a common notion of the CQI.